Old English sunne, from Proto-Germanic *sunnon (cf. Old Norse, Old Saxon, Old High German sunna, Middle Dutch sonne, Dutch zon, German Sonne, Gothic sunno), from PIE *s(u)wen- (cf. Avestan xueng "sun," Old Irish fur-sunnud "lighting up"), alternative form of root *saewel- "to shine, sun" (see Sol).
Old English sunne was fem., and the fem. pronoun was used until 16c.; since then masc. has prevailed. The empire on which the sun never sets (1630) originally was the Spanish, later the British. To have one's place in the sun (1680s) is from Pascal's "Pensées"; the German imperial foreign policy sense (1897) is from a speech by von Bülow.